Switched Reluctance Motors (SRMs) are commonly used in automotive applications, such as in window-lift devices, blowers, power seat adjustors, and hydraulic pump motors for anti-lock braking systems. SRMs possess advantages over brush-type motors, in the respects of (1) being more durable and (2) providing torque and speed characteristics which do not significantly change with temperature.
However, SRMs possess three disadvantages. One is that many of them do not deliver the maximum power which is theoretically available. A second is that they require large numbers of power transistors and power diodes, which are expensive. A third is that they produce audible noise. These disadvantages will be examined, with reference to FIGS. 1-5.
FIG. 1A illustrates a stator 3 in an SRM. Coils C.sub.A represent one phase of the motor. Similar coils are installed in the phases represented by dotted lines 6 and 9, but are not shown for ease of illustration. FIG. 1B shows a rotor 12 installed in the SRM.
FIG. 2 illustrates a prior-art switching circuit 14 for controlling current through the phases of the SRM. Inductance L.sub.A represents the coils C.sub.A, as indicated. Inductances L.sub.B and L.sub.C represent the inductances of the other phases 6 and 9 in FIG. 1A.
Assume that the SRM is running, and that rotor 12 is rotating counterclockwise, as indicated by the curved arrow in FIG. 3A. As pole P approaches coil C.sub.A, current is initiated in that coil preferably when pole P is approximately mid-way between coil C.sub.B and coil C.sub.A, that is, when angle A equals 30 degrees.
This energization is accomplished by turning transistors Q1 and Q2 ON in FIG. 3B, thereby allowing current I to flow through inductance L.sub.A, which represents coil C.sub.A. The turn-on of transistors Q1 and Q2 is accomplished by a control system known in the art, but not shown. An exemplary control system is the Harris Semiconductor gate-drive controller number D469A.
In order to obtain maximum torque from rotor 12 for the maximum angular rotation, the current in coil C.sub.A should then be terminated when pole P in FIG. 3A becomes perfectly aligned with coil C.sub.A, that is, when angle A equals zero. This termination is called "commutation." Termination should not, in theory, occur either before or after the aligned position is reached.
If termination occurs after alignment, the motor will begin acting as a generator, and will apply negative torque to the rotor 12, thereby reducing average power applied to the rotor. (From another point of view, after pole P reaches the aligned position, the still-energized coil C.sub.A tends to "pull" pole P backward.)
If termination occurs prior to the aligned position, maximum torque would not be applied through the entire rotor angle A shown in FIG. 3A, which is required to deliver maximum average power to the rotor 12.
However, it is not possible to apply full current through the full angle A in FIG. 3A, and then terminate the current instantaneously. The primary reason is that the inductance of coil C.sub.A has been steadily increasing since turn-on of transistors Q1 and Q2, because the air-gap between coil C.sub.A and the pole P has been decreasing. The inductance L.sub.A is relatively large when pole P approaches coil C.sub.A.
This large inductance prevents instantaneous termination of the current I in FIG. 3B. That is, in general, current in an inductance does not decay instantaneously, but decays exponentially. A large inductance causes the rate of decay to be slow, compared with the rate of decay for a small inductance. (In contrast, when the current was initiated in coil C.sub.A, as in FIG. 3B, the inductance of coil C.sub.A was significantly less. The delay imposed in initiating the current was correspondingly shorter, and, for present purposes, is considered negligible.)
Consequently, to allow for the delay required by the exponential decay, transistors Q1 and Q2 in FIG. 4 are turned OFF prematurely, before pole P reaches the aligned position, such as at angle A2. Upon turn-off, the current I decays through the path shown in the circuit, through diodes D1 and D2, and then either (a) returns to the battery B or (b) travels to one of the other inductors L, or a combination of (a) and (b).
The premature turn-off of transistors Q1 and Q2 can reduce power delivered by the motor from the theoretical maximum, as will be shown by computing the angle A2 at which turn-off occurs in the prior art.
In practice, in many automotive SRMs, the discharge time for inductor L.sub.A in FIG. 4 lies in the range of 400 micro-seconds. A motor running at 3600 rpm requires 1/60 second, or 16,667 micro-seconds, to complete a single revolution. At this speed, 400 micro-seconds represents 2.3 percent of one revolution, or 8.3 degrees.
If angle A in FIG. 3A is 30 degrees, and if termination of the current must occur 8.3 degrees (which is angle A2 in FIG. 4) prior to the aligned position, then full current persists for only 22.7 degrees, compared with the 30 degrees which are theoretically available. Full torque is applied for only 22.7/30, or 75 percent of the rotor's travel.
Thus, the premature termination of the current, which is required by the inductance of coil C.sub.A, causes a reduction in output power of the motor.
Regarding the noise mentioned above, the coils C.sub.A in FIG. 4 produce magnetic field lines B. These field lines tend to "squeeze" the iron from which the stator 3 is constructed, and applies a type of hoop stress to the stator 3. However, when the current through the coils C.sub.A abruptly terminates in commutation, the "squeezing" also abruptly terminates, allowing the iron to relax, thereby creating an audible noise pulse.
The sequence of commutation events which occurs in a running motor causes a sequence of these noise pulses.
Therefore, the commutation circuit shown in FIG. 2 fails to deliver maximum power in the motor, and also produces noise. In addition, the circuit requires two transistors, plus two diodes, for each coil, for a total of six transistors and six diodes for the three-phase machine shown in FIG. 4. These components are somewhat expensive, because they must be designed to withstand (a) relatively high currents, and (b) relatively high voltage surges during the commutation cycle.